1. Field of the Invention
The present invention relates to differential amplifiers, including instrumentation amplifiers, and in particular, differential amplifiers requiring high signal gain and wide input signal range.
2. Description of the Related Art
In many applications for differential amplifiers, particularly instrumentation amplifiers, it is often a design requirement to provide multiple gain settings, in one or more of which it is often necessary to provide maximum loop gain to guarantee adequate gain accuracy. One or more of the remaining gain settings may not require the loop gain to be as high, but it may be necessary to accommodate a much wider input signal range than might be available at the higher gain settings. These requirements are generally conflicting and are often resolved only by increasing the complexity of the circuitry.
For example, in an amplifier having two gain settings, e.g., 10 and 1,000, the maximum input signal range requirement for the higher gain setting may only be +/−5 millivolts. For the higher gain of 1,000, such a signal range will produce an output signal of +/−5 volts, which may be close to or at the maximum supply voltage VDD of the circuit. However, to maintain a gain accuracy of 0.1% at this higher gain setting, the overall loop gain of the system must be significantly higher, e.g., 1,000,000.
For the lower gain of 10, maintaining a gain accuracy of 0.1% would only require a loop gain of 10,000. However, with the input signal range of +/−5 millivolts, the output signal range will only be ±50 millivolts, which is a small portion of the available output signal range. In that case, a wider input signal range, e.g., +/−500 millivolts, may be more desirable. However, such a higher input signal range would be problematic for the higher gain.